The main objective of this proposed study is to develop a contraceptive drug combination which, when taken for 3 to 5 days close to the expected time of menstrual bleeding, will ensure normal physiological bleeding or, when taken during early pregnancy, will act as an abortifacient. We have already shown that prostaglandin F2 alpha given to estrogen-treated monkeys causes a precipitous drop in progesterone and initiation of menstrual flow. We now propose to test in the monkey the effect of other synthetic estrogens and progestins when given alone or in combination with prostaglandins or luteolytic agents like ergonovine and oxymethalone. An example of the combination treatment to be tested is the sequential treatment comprised of progesterone followed by prostaglandins or estradiol followed by prostaglandins. The total number of days of treatment will not exceed 5 to 6 days. These compounds will be administered during the luteal phase of the cycle and the effect on cycle length and blood progesterone levels will be determined. Treatments found effective during the cycle will be tried during early pregnancy. Prostaglandin will be measured in monkeys during the cycle, during pregnancy (with emphasis around the time of implantation), after estrogen treatment, and in hysterectomized and ovariectomized monkeys after estrogen or estrogen-progestin treatment. Elucidation of the steroid hormone secretion pattern through the menstrual cycle and pregnancy will be continued. Steroid hormones and prostaglandins will be measured by radioimmunoassay.